There is no deceleration in the WTC 1 acceleration graph, only greater or lesser
acceleration. However, in the Balzac-Vitry building demolition acceleration
graph there is real deceleration at 1.5 seconds into the fall. The velocity
in freefall after 1.5 seconds is 48.3 ft./s, so average velocity would have
been 24.15 ft./s during this interval. At that average velocity the roof of
the Balzac-Vitry building would have fallen 36.2 ft., which is the height of
three stories. There were three stories taken out for this demolition which
is obvious from the video. This puts the start of the deceleration right at
the impact of the upper intact block with the lower intact block.